Most of the rocks which survive are volcanic (igneous) and metamorphic in origin. Volcanic activity was everywhere. When the Archaean began, the Earth's heat flow was nearly three times higher than it is today. It was still twice the current level when the Archaean ended, 2,500 mya. This high level of heat flow may have made plate tectonics more vigorous than today. The question of when plate tectonics began is a major research area.[3] The large supercontinentVaalbara formed in the Archaean. There were oceans already, before the Archaean. The atmosphere almost entirely lacked free oxygen, and instead, it was mostly methane and CO2.